Italian VLF design

[dom44]

I was on an obscure forum in metal detecting when I came across this design I have just built it after redesigning the PCB the image is not very clear but it worked first go and is an easy detector to build
I have not tested it yet in the field, I am building it into a small hand held detector for checking small patches for nuggets or to search up walls.
My search coil is only 100mm diameter but shows good sensitivity I used an ne5534 for the coil amp, and a tl074.

[waltr]

PCB looks nicely done.

Schematic is a little hard to read. Any better image available to post.

What frequency does the TX run at and what are to coil specs.

[dom44]

I will have to redraw it give me a bit of time it doesn't say the frequency just gives the transmit coil inductance and that is a bit hard to read i would say anywhere around 15khz, my coil was putting out about 50khz on the bench while i was playing around with it and was still showing signs fer rite reject with ground balance and iron discriminate showed signs of working as well but i have to play around with it to bring it down in frequency as i am using a 100mm coil, but picks up a mans ring at about 160mm with that coil. important thing to note is there is a 1meg resistor in parallel with D2 on the board that is not marked on the schematic but you can see it on the photo.
this is the original parts placement.
if you want the sprint layout i redrew let me know.



also just found this info while searching

[dom44]

Here is the redrawn schematic I think it should be right

correct one others have minor errors.....

[ivconic]

Can't anyone tell it is a Tesoro?

Long time seen on this forum already.
TGSL topic is HUGE and this version is sunk somewhere there.
If i recall correctly; i got it from Nakky or ApBerg, several years ago.

[dom44]

to hard to keep track of them all ivconic this version has a nice pcb and does work well
handy for a small portable unit.

[ivconic]

to hard to keep track of them all ivconic this version has a nice pcb and does work well
handy for a small portable unit.

Yes it is.
Whatever applies to TGSL; applies to this one too.
Since i got it from Nakky or ApBerg; i would call it "Dutch version", although "Sceatta" resembles on Italian.

[ivconic]

"Drawn: Ap".
So it is ApBerg.
My memory serves me well after all.
From time to time...

[dom44]

I thought it was very clever the way they used the negative voltage generator to generate the tone as well keep the part count down, that's what caught my eye first and threw me off from working out it was TGSL

[kt315]

tesoro pantera-royal sabre-GMT king cobra, amputated certainly to tesoro amigo-compadre. new? not.
all you say thats new is long forgotten old. negative voltage generator was to generate TWO tones, in right full schematic.
that we call NOTCH FILTER, that was insolently cutted in this 'new light tesoro one'.

PS. this is sure tesoro silver sabre I.


Re: Silver Sabre circuit ***IMPROVEMENTS***
Posted by Sean Goddard
Wednesday 10 April 2002, 11:33:00
In Reply to Re: silver saber circuit - Posted by Carl on 4/04/02

You could try different Op-Amps in the SS circuit, but if you look LONG and HARD at the existing ones, you wont find any which are better them in the areas which matter most.

You COULD try ones like the National Semiconductor LF353N (MUST be the N, as it's an "improved" version of the standard LF353.

I believe there's a newer version of the '442 as well which has better a current noise figure.

The "bottom" signal path has a constant retune and Isuspect is the "motion" path. Changing the time constants of the two "auto balancing" circuits i.e. the LF442 and the LM358 blocks, simply involves changing BOTH the 330K resistors.

You could try starting at 470K. This will "theoretically" give you better sensitivity to small items. Too high a value, and you will "smear" the motion signal.

Also, changing the value of the two 150K resistors on the "front end" LF353 to 330K will give a marginal improvement in depth and sensitivity, but too much will make the GB setting a LOT more critical, as well as making the detector VERY unstable.

Replacing the TIS75 VERY CRUDE synchronous demodulators with REAL "full time" demodulator circuits which also sample the negative portion of the target signal, will SIGNIFICANTLY reduce the system noise levels and drift, and make a much more stable machine.

Other methods are to amplify the incoming signal, and phase shift the ground and discriminate signals DIRECTLY, this means that you could dispense with the "wandering" Colpitts type TX oscillator in the top lefthand corner, and drive the thing with a crystal (VERY STABLE) one. Using a buffered take off of the coild drive signal for the Demodulator switching signal also reduces noise as it eliminates clock "jitter".

Finally, use "metal oxide" resistors THROUGHOUT, they're a LOT quieter than the cheap carbon ones used by Tesoro, and use OFC cable to do ALL the link-up. Keep ALL cables as SHORT as possible, and use screened cable if you can, only connect ONE END of the screen when running a single wire to controls, this will avoid creating multiple paths for any noise picked up on the screening, also connecting the screens at ONE COMMON point in a "star" ground will also help.

I could list over 30 simple and immediate improvements which took me just over two hours to come up with, but Tesoro aren't going to pay me for them.

Hope someone finds this info useful.
---
Sorry I don't have time right now to go into all the details of the improvements which have been suggested. Most of them will not improve anything, or will screw up things that are already good compromises.

The Silver Sabre circuit is one of the most successful beeper platforms in this industry. I think it dates to about 1986, and products which are very nearly the same thing are being manufactured today. Admittedly it is not particularly sensitive, but it is easy to build with readily available parts, and has an excellent discriminator.

I recommend building it just the way you see it on the schematic, using a purchased Tesoro searchcoil. If you happen to know how, you might want to substitute CMOS switches for the JFETS, which however is not likely to improve performance. If you're using a Tesoro coil of the specified type, I would discourage you from attempting to operate it at a different frequency or using a crystal controlled transmitter: although conceptually easy, there are many potential pitfalls in doing such things, which are not obvious.

Attempts to increase the sensitivity of this circuit are likely to either fail, result in only slight improvement, or cause other problems (which may not be obvious at first). If a person is striving for high sensitivity, the Silver Sabre is not a very good foundation for such a quest.

If, once you've gotten the circuit to work satisfactorily, you want to play around with it, the easiest thing to do is add simple "features", for instance an external ground balance knob and a switch to disable discrimination.

[dbanner]

Very nice PCB to stick into PVC pipe housing. Niced job dom44.
This is obviously a tesoro circuit. The TGS/royal sabre are very good detectors as far as sensititvity is concerned. This circuit is a stripdown of those. So performance will be comparable, if you can source a stock coil, even better.
The best part about this circuit is the long board layout. I love it.

[kt315]

The best part about this circuit is the long board layout. I love it.

longboard doing ApBerg underwater. Italia is not country where you can free hunting on beach. you must cover up
the detector in wine paper pocket/bag, on land, or in pants in case you are in cops' view, in water.

[dom44]

thanks kt315 excellent information i am sure it will be helpful and some of the tesoro circuits you posted i don't have thanks.

[dbanner]

longboard doing ApBerg underwater. Italia is not country where you can free hunting on beach. you must cover up
the detector in wine paper pocket/bag, on land, or in pants in case you are in cops' view, in water.

I see, so he had intended to conceal the device, hence the longboard design. He could hide it in his underpants, but where would he put the coil, up his a...?

[kt315]

DB seems we must refresh our loved topic on winter

https://www.geotech1.com/forums/show...etector!/page2